The DPT-1 is a multi-centered, randomized, national trial designed to determine whether antigen based therapy of high risk nondiabetic individuals can prevent subsequent IDDM. Insulin is administered IV for 4 days and then subcutaneously to those randomized to the treatment arm. The underlying hypothesis to be tested is that protection against clinical IDDM achieved by parenteral administration of insulin to subjects at high risk of clinical IDDM in the DPT-1 is mediated, at least in part, by insulin's metabolic effect to inhibit b-cell function (beta cell rest). Understanding the mechanism whereby parenteral insulin prevents IDDM would provide the basis for improved methods of prevention to be developed in the future.